1. Field of the Invention
Inorganic crystal ingots have been grown in Stockbarger furnaces on a commercial basis for many years. Pure crystals have found numerous optical uses in the medical, metal, military and other fields. Certain types of ionic crystals, particularly alkali metal and alkaline earth metal salts, when grown with a small amount of a specified impurity or dopant, have the ability to scintillate, or to give off measurable light when exposed to radiation. These scintillation crystals are used as radiation detectors for such applications as gamma ray spectrometry, geophysical surveys, and clinical detection of radioisotopes.
Crystals of these compounds are also grown with dopants to improve their hardness. This serves to avoid bending in the cleavage of X-ray diffraction plates and to enhance the yield strength of polycrystalline bodies formed therefrom.
In the growth of inorganic crystals in a Stockbarger or similar furnace, the starting material is placed in a crucible of platinum or other similar inert metal that does not react with the melt. The entire crucible is set on a vertically movable crucible holder mounted on an elevator located in the Stockbarger furnace. The furnace typically has two heating sections separated by a heat baffle. During growth, the top section of the furnace is heated to a temperature of 50.degree. C to 150.degree. C above the melting point of the inorganic crystal, while the bottom section of the furnace is maintained at a temperature slightly below the melting point. The crucible is initially placed in the top section, where the material is melted. Thereafter, the crucible is slowly lowered into the bottom section, whereupon the melt begins to crystallize, initially in the cone portion of the crucible. Solidification of the melt progresses upwardly through the crystal as the elevator is slowly lowered into the bottom, cooler section of the furnace.
One of the objectives of crystal growth in the Stockbarger furnace is to develop a single lattice structure. The success or failure of this objective depends upon a number of factors, including stratification of absence of thermal convection currents in the melt. Because of the much higher temperature of the liquid than the solid, there is a large flow of heat across the liquid-solid interface. Any stirring of the melt, such as that produced by convection, will disrupt the even flow of heat, change the growth rate, and cause flaws in the crystal ingot such as bubbles, or bands of high and low dopant concentration.
One problem that exists in connection with the intentional doping of inorganic salt crystals such as alkali metal halides, with scintillation dopants such as thallium, europium and sodium, is the phenomenen known as zone purification which occurs at the interface between the liquid and the solid. This phenomenen basically involves a tendency of the inorganic salt as it is crystallized to reject the impurities, which in the case of a scintillation crystal, is the dopant. As a result, the dopant becomes increasingly concentrated in the liquid at the interface. This tendency is magnified during crystal growth in a Stockbarger furnace by the fact that the large quantum of heat flowing from the liquid to the solid contributes to the ability of the inorganic salt, upon crystallization, to exclude impurities. In addition to the dopant, dissolved gases are rejected by the inorganic salt as it solidifies and tend to collect in the liquid at the crystal growth interface.
A well grown crystal ingot is composed of a single crystal lattice, or more typically, a mosaic of crystal domains so closely aligned that the behavior of the body approximates a single crystal lattice. However, when growing larger crystals of 10 inch diameter on up, it is very difficult in practice to obtain a single crystal. Instead, the ingot is composed of a plurality of macrocrystal components held tightly together. With pure materials crystallizing in the cubic system there generally are no reflections or diffraction of light at these boundaries between components, so optically single bodies are cut from these ingots which from a lattice structure viewpoint are polycrystalline. During crystal growth, any abnormality in the growth procedure is likely to generate a crack, fissure or fault which continues or propogates through the ingot as the growth proceeds, and which becomes a boundary between adjacent macrocrystals within the ingot. The dopant or any impurities that are concentrated in the liquid melt at the interface tend to collect at these boundaries between adjacent macrocrystals. This results in non-uniformity of the crystal with a relatively higher percent of dopant at the boundaries than throughout the body of each macrocrystal. The higher amount of dopant at the boundary has a tendency to lower the melting point of the crystal at these boundaries to a lower temperature isotherm, sometimes by 20.degree. C or more. This tendency is more pronounced for dopants that form a low melting eutectic with the host crystal.
After complete solidification, the crucible containing the crystal ingot is normally removed from the growth furnace and is inverted and placed in a melt-out furnace which is heated to a temperature considerably above the melting point to separate the ingot from the crucible. This brief exposure to high temperatures, lasting no more than a few seconds, is often sufficient to melt the dopant-rich solid along the boundaries between the macrocrystals and to allow some of the dopant-rich melt to flow out of the boundaries, leaving open boundaries or cracks between the macrocrystals, and disrupting the integrity of the ingot. This seriously reduces the amount of the crystal that can be cut into usable products and limits the sizes available from a given crucible or furnace.